1. Field of the Invention
The present invention relates to a lock-in amplifier and, more specifically, to a heterodyning lock-in amplifier.
2. Description of the Related Art
A lock-in amplifier is an extremely selective ac voltmeter used to measure a single-frequency signal obscured by noise. Operating over input signals ranging from 0.1 Hz to 200 KHz, it rejects noise, interference and harmonics of the measurement frequency in the input signal, producing a dc output proportional to the rms amplitude of the sinusoidal fundamental of the signal.
A lock-in amplifier behaves as if it were a narrow bandpass filter followed by an rms meter which rectifies the "purified" sinusoidal signal to a smooth dc output. Actually, however, it performs these processes in reverse order: first rectifying, then filtering. To rectify, a reference frequency (F.sub.R) having the same frequency as the frequency of interest in the input signal is mixed with the input signal. This downshifts the signal input spectrum by exactly F.sub.R, causing the frequency of interest to appear as a pure dc output level. All other components (e.g. noise) in the input signal appear as ac fluctuations, which can be easily filtered out by a simple RC low pass filter.
A typical prior art lock-in amplifier is shown in FIG. 1. The input signal is initially sent through an ac amplifier 2 and a filter 4. Amplifier 2 brings the input signal up to a level adequate for subsequent processing. Filter 4, which can be switched in if desired, serves to remove signal components which would cause errors in subsequent processing, such as odd harmonics of the input signal (3F.sub.R,5F.sub.R, . . . ). It also increases the tolerance of the circuit to high levels of interference (dynamic reserve) by excluding wideband components of the input spectrum and thereby conserves headroom.
The reference frequency signal F.sub.R is first conditioned and stabilized by a phase locked loop 6. A phase sensitive detector (PSD) 8 is used to multiply the input signal waveform by the reference signal waveform. The output of the PSD is maximum when the reference signal is in phase with the input signal. A phase shifter 10 is provided to adjust the phase of the reference signal to bring the two signals into alignment. The output of PSD 8 is a signal with a dc level corresponding to the signal magnitude of the frequency of interest in the input signal. The ac component in the signal output from PSD 8 corresponds to unwanted frequencies in the input signal, e.g. noise, which are then filtered out by an adjustable low pass filter 11. An adjustable amplifier 12 is provided to obtain a dc signal output on the order of volts.
In a heterodyning lock-in amplifier, the frequency spectrum of the input signal is first upshifted to a fixed intermediate frequency (F.sub.0), then filtered by a fixed narrow bandpass filter which only passes signals at F.sub.0, then downshifted to dc by the PSD. In this circuit, the PSD always operates at a fixed frequency F.sub.0 rather than at a variable frequency F.sub.R (F.sub.R being varied so as to coincide with the frequency of the particular input signal). For this reason, a heterodyning lock-in amplifier is very stable and insensitive to harmonics of F.sub.R.
A typical heterodyning lock-in amplifier is shown in FIG. 2. To accomplish the upshifting in frequency, the reference signal (frequency F.sub.R) is combined in mixer 12 with fixed intermediate frequency (F.sub.0), producing a signal having a frequency F.sub.0 +F.sub.R. This signal is multiplied with the input signal (frequency F.sub.R) in mixer 14 to produce an upshifted output F.sub.0. This signal is sent through a narrow band pass filter 16 operating at F.sub.0 to remove all extraneous frequencies, then multiplied by F.sub.0 in PSD 8. As before, a phase shifter 10 is provided to align the phases of the two inputs to the PSD 8. The output of PSD 8 is then low pass filtered as before, yielding a dc output proportional to the magnitude of the signal of interest in the input.
The heterodyning design for a lock-in amplifier is an improvement over the standard lock-in amplifier design because the PSD always operates at the same frequency F.sub.0 regardless of the input frequency. However, F.sub.0 must be a relatively high frequency so that the phase locked loop operates in a high lock range. This in turn requires an expensive narrow band pass filter 16, since that filter must operate at the same high frequency F.sub.0. Thus, heterodyning lock-in amplifiers are more accurate than simple lock-in amplifiers, but are also more expensive.